In injection molding applications, cavity pressure sensors are used to provide process data to facilitate improvement in part quality and to detect quality defects automatically. Cavity pressure sensors consist of a sensor body, which creates an electrical signal from an applied force. They also include a cable which transfers that electrical signal to the signal conditioning unit. Signal conditioning, which amplifies the analog electrical signal, and in some instances converts the electrical signal to a digital signal to be read into a computerized process monitor or controlling device.
Cavity sensing for injection molding is based on two technologies, piezoelectric and strain gage sensors. In the piezoelectric sensors, the sensor body creates an electric charge, measured in Pico Coulombs, which is carried to the signal conditioning via a two conductor coaxial cable. For nearly a decade, piezoelectric sensors have been available with multichannel connectors. The advantage of these multichannel connectors is a single connection point that allows multiple sensors to be attached to their respective signal conditioning units. This reduces the space required on the side of the mold, the number of cables required for connection to the signal conditioning, and the complexity involved with matching multiple sensors to their respective signal conditioning units.
Also, in many injection molding sensor applications, signal conditioning is mounted directly on the side of the mold. While this provides certain advantages, it subjects the signal conditioning to high heat, shock, and impact on some molds, which can permanently damage the signal conditioning unit. With multichannel connectors, signal conditioning units can be mounted away from the heat and shock of the mold, while utilizing only a single connecting cable from the mold to the signal conditioning.
Strain gage sensors, on the other hand, require cables with four conductors: two of the connectors providing excitation voltage and two that carry the millivolt signal. These sensors have integral cables, which in most cases are permanently attached to the signal conditioning unit. Customers would like to be able to use multichannel connectors in strain gage sensor applications, but this has been limited by the following technological challenges.
First, in the strain gage sensors, the millivolt signal carried by the cable requires electronic shielding to prevent signal noise from, being introduced by stray electromagnetic interference (EMI). EMI is a particular problem in injection molding environments, and strain gage sensors for molding applications have historically required shielding to prevent signal noise.
Secondly, the multichannel connector requires connection points for the sensor to attach to the multichannel connector. Because of the higher number of conductors, it is difficult to find a connector that is small enough to fit onto a multichannel connector. A major limiting factor in the size of the connector is the metal shell which is normally used to provide shielding against electronic noise.
Also it is difficult to find a shielded connector that reliably carries the cable's shield through when connected.
Finally, a connectors that meet both size and shielding constraints are extremely expensive.
The present invention overcomes all of these challenges through the use of unshielded cables and connectors inside the mold. By removing the shielding, the inventors herein have discovered that smaller, less expensive connectors can be used to attach individual strain gage sensors to a multichannel connector. Also, in order to prevent introduction of signal noise due to EMI, it was discovered that signal noise reduction is accomplished by tightly enclosing the unshielded cables and connectors within a tightly sealed shielding enclosure inside the mold. Here, the mold and the multichannel connector, tightly sealed, provide this shielding. A shielded extension cable, with its shield tied 360° to the multichannel connector, is then used to carry the signal to the signal conditioner, which can be mounted on or off of the mold, away from heat and shock.